A. Field of the Invention
The present invention relates to methods, articles and apparatus for comfortably supporting a seated or recumbent human body. More particularly, the invention relates to a method and apparatus for minimizing concentration of forces on supported body parts using an adaptive cushion having a matrix of air bladder cells which are dynamically pressurized in response to measurements of body forces exerted on sensors overlying the cells.
B. Description of Background Art
Whenever a human body is supported by an object such as a chair or bed, normal and shear forces produced in reaction to the weight of the individual are transmitted from the supporting surface through the skin, adipose tissues, muscles, etc. to the skeleton. The forces exerted on body parts by support surfaces, which are equal and opposite to body weight forces, can in some cases cause damage to tissues. Forces on body parts can compress internal blood vessels and occlude nutrients from the tissue, the product of the magnitude and duration of these forces determining whether tissue damage or morbidity will occur. High pressure alone is generally not sufficient to deleteriously affect tissue. Deep-sea divers for example, are subjected to high, but evenly distributed normal forces and do not suffer from tissue damage. If, however, there is a sufficiently large external pressure gradient on a body part, resulting from, for example, a low-pressure area adjacent to a high-pressure area, internal body fluids can migrate to the area of lower pressure. Tangential or shear forces exerted externally on a body part can also collapse internal capillaries and blood vessels by distorting them along their longitudinal axes. It is therefore extremely important to know both the surface force gradient (pressure gradient) and the externally applied shear force exerted on tissue, because it is the combination of these factors that leads to tissue strain and subsequent tissue death. Thus, even relatively small external shear and normal forces, which may be independent of one another, can combine to produce damagingly large shear stresses on internal tissue. The areas of the human body which are most at risk of developing tissue damage such as a pressure sore are: heel, ischial tuberosities, greater trochanter, occiput and sacrum.
There are available a variety of pressure/force sensors, shear sensors and sensor arrays which are useable for measuring normal and shear forces exerted on human tissues. For example, the present inventor's U.S. Pat. No. 5,751,973, Nov. 5, 1996, Multi-Directional Piezoresistive Shear And Normal Force Sensors For Hospital Mattresses And Seat Cushions discloses thin, planar sensors for measuring reaction forces exerted by mattresses or chair pads on the body of a recumbent or seated patient. One embodiment of the invention disclosed in the specification of the ‘973 patent includes a sensor comprised of a two-dimensional array of isolated sensor element pads, each consisting of a thin, flat layer formed of a non-conductive elastomeric polymer matrix filled with electrically conductive particles.’ A matrix of upper and lower conductive elements in electrical contact with upper and lower sides of each sensor pad enables separate measurements to be made of the electrical resistance of each pad. Pressure exerted on each pad, e.g., in response to a normal force exerted on the sensor matrix by a person's body, reduces the thickness of the sensor pad, and therefore its electrical resistance by a bulk or volume piezoresistive effect.
The present inventor also disclosed a novel method and apparatus for measuring pressures exerted on human feet or horses' hooves in U.S. Pat. No. 6,216,545, Apr. 17, 2001, Piezoresistive Foot Pressure Measurement. The novel apparatus disclosed in the “545 patent includes a rectangular array of piezoresistive force sensor elements encapsulated in a thin, flexible polymer package. Each sensor element includes a polymer fabric mesh impregnated with conductive particles suspended in an elastomeric matrix such as silicone rubber. The piezoresistive mesh layer is sandwiched between an array of row and column conductor strip laminations, preferably made of a nylon mesh impregnated with printed metallic paths. Each region of piezoresistive material sandwiched between a row conductor and column conductor comprises an individually addressable normal force or pressure sensor in a rectangular array of sensors, the resistance of which varies inversely in a pre-determined way as a function of pressure exerted on the sensors, and thus enabling the force or pressure. distribution exerted by an object contacting the array to be mapped.
In U.S. Pat. No. 6,543,299, Apr. 8, 2003, Pressure Measurement Sensor With Piezoresistive Thread Lattice, the present inventor disclosed a transducer sensor array for measuring forces or pressures exerted on a surface, the array including a fabric-like, two-dimensional lattice of individual force or pressure sensor transducer elements comprising intersecting regions of pairs of elongated, flexible threads, each consisting of a central electrically conductive wire core covered by a layer of piezoresistive material which has an electrical resistivity that varies inversely with pressure exerted on the material.
In U.S. Pat. No. 7,201,063, Apr. 10, 2007, Normal Force Gradient/Shear Force Sensors And Method Of Measuring Internal Biological Tissue Stress, the present inventor disclosed a normal force gradient/shear force sensor device and measurement method for measuring internal stresses in tissues of a person supported by a chair or bed. The device includes a planar matrix array of peripheral normal force sensors radially spaced from central shear force sensors, each including an electrically conductive disk located within a circular opening bordered by circumferentially spaced apart electrodes. The disk and electrodes are located between upper and lower cover sheets made of a stretchable material such as polyurethane, one cover sheet being adhered to the disk and the other sheet being adhered to a support sheet for the electrodes. Motion between the cover sheets in response to shear forces exerted on the array causes the disk to press more or less tightly against the electrodes, thus varying electrical conductance between the disk and electrodes proportionally to the magnitude and direction of the shear force. Each normal force sensor includes an electrically conductive film pressed between row and column conductors. Measurements of conductance values of pairs of sensor, which vary proportionally to normal forces exerted on the sensor, are used to calculate a gradient vector of normal forces exerted by a body part on the sensor array, which is combined with the shear force vectors in an algorithm to calculate internal reaction shear forces, e.g., on flesh near a bony prominence.
The first group of the present inventor's patents identified above disclosed shear and normal force sensors and arrays which are useful in producing maps of normal and shear forces exerted at discrete points on a surface, such as a human body part, by an object such as the supporting surface of a chair or bed. The last of the present inventor's patents identified above provided an effective means for measuring shear forces and stresses on human tissue which is located some distance below the surface of the skin.
In U.S. Pat. No. 6,721,980, Force Optimization Surface Apparatus And Method, the present inventor and co-inventors disclosed an apparatus including a mattress which included a plurality of laterally disposed, tubular sausage-shaped air bladders, each having thereon an individual force sensor. The apparatus included a mechanism for individually inflating each of the air bladders, monitoring the pressure in each individual bladder while a person was lying on the mattress monitoring the force exerted on that particular bladder, adjusting the pressure of that individual bladder for the purpose of minimizing force exerted by that particular bladder on the person's body, and repeating the foregoing steps for each bladder cell in turn.
The method described in U.S. Pat. No. 6,721,980 of measuring force exerted by a person's body on a single individual air bladder cell while adjusting the inflation pressure in that cell may be suitable for single air bladder systems, and for those conditions in which the body of a supported patient freely conforms to the support surface. However, for the more frequently encountered cases in which portions of a patient's body are straddled between and supported by adjacent air bladder cells, the force measured on a particular bladder whose air pressure is bing adjusted may be minimal for a particular inflated pressure. But the pressure which may minimize force exerted on a particular air bladder cell will in general not be the optimum pressure for minimum total force concentrations on a person's body. This is because while the force exerted on a particular air bladder cell may be minimized, forces exerted on air bladder cells adjacent to the air bladder cell in which the pressure is being varied may be substantially increased because the load weight is shifted to the adjacent cells.
A similar limitation of the prior art methods and apparatus occurs when a portion of a patient's body is supported in a cantilevered manner from one or more adjacent air bladder cells while pressure is varied in a particular air bladder cell. Again in that case, load forces are transferred to adjacent air bladder cells. Accordingly; it would be desirable to provide a method and apparatus which accounted for all forces exerted on all air bladder cells while varying pressure in any individual cell The present invention was conceived of to provide a method and apparatus for minimizing body force concentrations on parts of a human body supported by a chair or bed cushion, which includes measuring forces exerted on body parts.